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43 Merc dieseling problem

dave_c

New member
"I have a 2000 Maxum with the

"I have a 2000 Maxum with the 4.3 merc in it. Every time i am on the water just long enough to heat up the engine and go to turn it off, it diesels real bad and i have to let it idle for a few minutes to stop the engine. I use speedway 87 octane gas and it runs great, but what can I do to stop that."
 
"If it is not already,set the

"If it is not already,set the idle down to 600rpm and use a better grade of fuel. Check the idle control solenoid if the engine has one."
 
Thanks alot. I also was told t

Thanks alot. I also was told that using Speedway gasoline will do more harm than good because of the alcohol that is in their fuel. Speedway says that there is less that 10% alcohol in any grade of fuel. Is this a true statement?
 
"They show less than 10% on th

"They show less than 10% on their pumps and by law, that's what it's supposed to have. Here in MKE, the Speedway close to my house goes through a lot of gas so it really doesn't get old. Alcohol burns cooler than gasoline but I would try 89 octane if you still have the problem after re-setting the idle.

Look for a thread titled "Change carb to injection" or something close to that. I posted a list of contributing factors and possible remedies, from a Mercruiser service bulletin I had faxed to me from their technical department. The dealer I worked at had the same bulletins but when we all left due to a management meltdown, I couldn't take copies with me. The idle solenoid was separate from the list and I posted the part numbers with the price

I'm not aware that Mercruiser ever added the solenoid to their motors as a factory installed part but if you want, call Maxxum and see if your boat had any factory recalls and get a list."
 
Just a second opinion. Stay aw

Just a second opinion. Stay away from Alcohol and use higher grade gas. Let us know if the detonation continues. Make sure your timing is at specs. Not overheating is it?
 
"David- this has been a proble

"David- this has been a problem for Mercruiser motors since they made some changes for the '97 model year. They put out a bulletin in '97 and revised it in '99, and I posted the main points of it in the thread mentioned in my previous post. Ethanol didn't seen to make a difference when the problem first began and I don't think it'll make a difference now. Besides, back in '97, they were adding MTBE and going to non-reformulated gas didn't help, either. Higher octane may help, though.

It's actually not a detonation issue, it's run-on and I haven't heard an engine with the problem in this (and other recent threads) detonating, even in hot weather at WOT.
"
 
"Thanks for your nomenclature

"Thanks for your nomenclature correction, Jimmy. yes... run on - not detonation. My opinion to stay away from Ethanol is independent of the run-on problem."
 
"If there is no ignition to th

"If there is no ignition to the engine and it continues to run it is dieseling.
There is no differance in dieseling / detonation from run on. Only a different term.
An engine must have ignition to run.
If the key is off there is no spark so if the engine continues on after it is turned off it is from the fuel being ignited from excess heat and compression of the fuel. Call it run on ,dieseling,detonation its all the samething. Fuel burning from heat and compression with no outside sorce of ignition like a spark plug.
Just as a diesel engine works.
The only way to stop a diesel engine is to remove the fuel supply.
Newer diesels have the pump shut down when the power is removed,older diesels had a mechanical valve operated by a remote cable or lever that closed the fuel supply valve ending the fuel supply. The older diesels were slow to stop. They needed to use the remaining fuel first. Newer diesels with electronic controls stop much faster.
The gas engine will continue on because the engine can still pull fuel through the carburetor and has a fresh supply of fuel being given the carb by the mechanical fuel pump.
Engines with electric fuel pumps will not continue very long because the pump stops giving fuel when the ignition is turned off.
This is why the idle control was used.
It helped stop fuel from entering the intake by closing the throttle valve stopping the air supply needed to pull the fuel into the intake and making it so the engine could not idle.
When the ignition is on this idle control is what sets the engine idle speed by holding the throttle valve open.

Better / higher octane fuel will burn slower and needs more heat to burn. This is why it helps stop dieseling. Higher octane has a higher flash point this is why it is used in higher compression engines.
Combine heat and high compression and the fuel tries to combust before the plug fires.(pre-ignition)
The higher flash point allows the fuel to keep from burning before the plug fires."
 
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